68 MARIE BENTIVOGLIO. 



crystal); very narrow h face; large m face; prominent 

 basal pinacoid with rough surface and giving poor reflec- 

 tions; and small z (231): — are also the chief features which 

 characterize all the twinned crystals observed from this 

 locality. Owing to twinning, however, the complete 

 development of individual crystals has often been checked. 

 Extra faces which occur on some twins, and which are 

 absent from the type crystal are the prisms k (340) and 

 A (780), sometimes replacing and sometimes occurring 

 with r (230), B (570) and (540); and the pyramid t (133), a 

 small face giving indefinite signals. (For frequency of 

 occurrence of faces, see table of distribution of forms). 



The common form of doublet developed is that shown in 

 figs. 3 and 3a. Both segments are about the same size and 

 equally developed. The twin plane is distinct, but reentrant 

 angles are not very marked. In other similar doublets the 

 t (133) and c (001) faces are absent, and so the reentrant 

 angles are still less marked. A noteworthy feature is that 

 certain z (231) faces of the normal crystal, instead of 

 remaining small in size, often become elongated and 

 narrow, running parallel to the twin plane. Occasionally 

 only one individual of the twin is well developed (fig. 4),, 

 then the junction line between the two crystals follows an 

 irregular course between the basal pinacoid of one crystal 

 and the a face of the other segment. Thus the more 

 developed crystal appears to arch over the other segment. 



Sometimes, again, one or two small individuals are seen 

 in twin position projecting from the end of a stoutly pris- 

 matic crystal, (fig. 5). The s (111), z (231) and e (101) 

 faces are well developed on the large normal crystal and the 

 twin segments projecting from these are clearly seen. In 

 fig. 5 the third individual is extremely small and twinned 

 on e (101), while the second larger segment is twinned on 

 e (101) of the normal prismatic crystal. 



